<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<body>
  <button id="btn">点击</button>  
  <div id="demo" style="display: none">这是是个惊喜！</div>
  <script>
      function $(id){
          return document.getElementById(id);
      }
      function show(){
          $('demo').style.display = 'block';
      }
      $('btn').onclick = show;
  </script>
</body>
</html>